This invention relates to an integrated power plant comprising a molten carbonate type fuel cell using a carbonate such as sodium carbonate, potassium carbonate, etc. in a molten state as an electrolyte, a gas turbinegenerator and a steam turbine generator, and a method for operating the plant.
A fuel cell using a carbonate such as sodium carbonate, potassium carbonate, etc. in a molten state at 600.degree. to 700.degree. C. is called a molten carbonate type fuel cell. The molten carbonate type fuel cell is operated at such a high temperature that the reaction proceeds actively and no expensive catalyst such as platinum, etc. is required. In the molten carbonate type fuel cell, a hydrogen gas or a hydrogen-containing gas is supplied to the anode as a fuel gas, and a mixture of air and carbon dioxide gas is supplied to the cathode. The air and carbon dioxide gas receive electrons at the cathode and enters into the electrolyte in the form of carbonate ions. On the other hand, hydrogen reacts with the carbonate ions in the electrolyte at the anode to form a carbon dioxide gas and water and emit electrons. As a result, the molten carbonate type fuel cell consumes the carbon dioxide gas at the cathode, and produces the carbon dioxide gas and water at the anode. Thus, in the molten carbonate type fuel cell, when a utilization ratio of fuel gas (amount of fuel gas actually consumed at the anode/amount of fuel gas supplied to the anode) is increased, the fuel gas is diluted with the carbon dioxide gas and the water formed at the anode. At the same time, the consumption of fuel gas proceeds owing to the reaction in the cell, and thus the concentrations of the components taking part in the reaction of the fuel gas are considerably reduced, thereby lowering the output voltage of the fuel cell at a given current density and thus lowering the thermal efficiency. That is, with increasing utilization ratio of a fuel gas, a net fuel cell efficiency is lowered. The net fuel cell efficiency is defined by [cell output (kW).times.860]/(heat of reaction of fuel gas actually consumed at the anode), and thus the fuel gas efficiency of fuel cell is lowered with increasing utilization ratio of fuel gas.
A nominal fuel cell efficiency is defined by [cell output (kW).times.860]/(heat of reaction of fuel gas supplied to the anode) and can be obtained as (net fuel cell efficiency).times.(utilization ratio of fuel gas). Thus, in order to increase the thermal efficiency of a fuel cell power plant, it is necessary to develop a fuel cell proper with less reduction in net fuel cell efficiency against increasing utilization ratio of fuel gas and to develop an integrated power plant with a higher thermal efficiency based on a higher overall heat recovery against a low utilization ratio of a fuel gas in the fuel cell.
It is known that the thermal efficiency of a fuel cell is increased with increasing pressure of a fuel gas. A fuel gas to the anode now widely used is a reformed gas composed mainly of hydrogen and carbon monoxide, obtained by reforming a natural gas under a pressure of 6 to 10 kg/cm.sup.2 in a reformer, followed by heating to about 600.degree. C. through a heat exchanger.
A gas turbine combined plant (gas turbine +steam turbine) now practically used has an arrangement shown in FIG. 1.
A gas turbine 29 is composed of a compressor 30, a combustor 32 and a turbine 31, where atmospheric air 23 is compressed by the compressor 30 and a fuel gas 1 is combusted together with the compressed air in the combustor 32. The combustion gas is led to the turbine 31 to convert the thermal energy to a mechanical energy which drives a generator 33 to generate electric power. The temperature of the combustion gas at the inlet of the turbine 31, now practically used, is as high as 1,000.degree.-1,100.degree. C., and consequently the exhaust gas 24 from the turbine 31 has a temperature as high as 500.degree.-550.degree. C., and thus can be utilized to generate steam 26 in a waste heat recovery boiler 36 and then vented to the atmosphere as a vent gas 25 having a thoroughly low temperature. The pressure of steam 26 can be selected to optimize the steam cycle, and usually ranges in 40-70 kg/cm.sup.2. The steam 26 is fed to a steam turbine to drive the generator 33 common to the gas turbine 29 to generate electric power.
The effluent steam from the steam turbine 34 is cooled in a condenser 35 with cooling water such as sea water, etc. and fed to the waste heat recovery boiler 36 as boiler feed water 27 to form a circulating system.
In FIG. 1, an example that the steam turbine 34 and the gas turbine 29 drive the common generator 33 to generate electric power is shown, but the steam turbine 34 and the gas turbine 29 can drive their own individual generators. Steam 28 bled from the steam turbine 34 is injected into the combustor 32 for abating the amount of NO.sub.X, and can be generally injected up to about twice the flow rate of the fuel gas 4. The fuel gas 4 at the ordinary temperature is supplied to the combustor 32 and combusted at a temperature of 1,000.degree. to 1,100.degree. C. In the country having a severe environmental control over NO.sub.X, a NO.sub.X removal means is provided in the waste heat recovery boiler 36.
In the conventional integrated power plant using a fuel cell, a cathode off-gas is proposed to be used as a turbine driving gas, but the temperature of a cathode off-gas is 700.degree. to 750.degree. C., which is lower than that of the gas supplied to the turbine now used and lowers the thermal efficiency of the turbine. Furthermore, the temperature of turbine off-gas is as low as 300.degree.-400.degree. C., and the steam generated in the waste heat recovery boiler with such turbine off-gas is not enough for driving the steam turbine.
Japanese Patent Application Kokai (Laid-open) No. 61-39459 discloses that a combustor is provided in a conduit for leading a cathode off-gas to an expansion turbine in a turbo-charger for compressing and supplying air to the cathode of a fuel cell, and a portion of anode off-gas is led to the combustor and combusted together with the cathode off-gas as a combustion air. The combustion gas at a higher temperature from the cumbustor is led to the expansion turbine, thereby increasing the overall plant thermal efficiency. In this molten carbonate type fuel cell, it is necessary to recycle CO.sub.2 to the cathode, and a portion of the said combustion gas is separated, compressed and added to the air to the cathode.
Japanese Patent Publication No. 58-56231 discloses a power plant using a phosphoric acid type fuel cell, where unreacted fuel component in the anode off-gas from the fuel cell is combusted in a reformer burner to supply a portion of the necessary heat for reforming the fuel gas and also to utilize the resulting combustion gas as a thermal energy to drive an expansion turbine of the turbo-charger, thereby driving a compressor of the turbo-charger for supplying air to the cathode of the fuel cell. That is, the air compressed by the compressor of the turbo-charger is supplied to the cathode of the fuel cell and the reformer burner, and the burner combustion gas and the cathode off-gas are led to the expansion turbine of the turbo-charger to recover the power for driving the compressor of the turbo-charger.
In the foregoing art, the chemical energy of a fuel gas is converted to an electric energy as much as possible in the fuel gas, and the fuel component in the anode off-gas of the fuel cell is utilized in the combustion in the reformer burner or the CO.sub.2 formation recycle and ultimately in the power recovery of the air compressor of the turbo-charger for supplying air to the cathode of the fuel cell by driving the expansion turbine and in cooling the cathode of the fuel cell by the air, or also in electric power generation, if necessary. In this case, the power generation by the expansion turbine is auxiliary and its power ratio to the electric power generated by the fuel cell proper is only a few to ten-odd percent.
When the scale of a power plant based on a fuel cell is small, the capacity of expansion turbine-generator is relatively very small, and the scale merit as a rotor cannot be utilized and the mechanical loss becomes larger. Thus, the provision of expansion turbine-generator is not economically effective. In other words, a power plant will generally become a type of relatively low effect using only a turbo-charger.
As a means for increasing the power recovery in the expansion turbine relatively to the power of a fuel cell, the temperature and pressure of a gas to the expansion turbine are increased or additional combustion of a fuel by providing an auxiliary burner and supply of the combustion gas to the expansion turbine have been proposed, as described above, but the temperature and the pressure of the gas to the expansion turbine depend upon the operating temperature and pressure of the fuel cell proper and thus new tasks for development are imposed on the fuel cell proper. The addition of the auxiliary burner requires a new technical development for combusting the anode off-gas of low calorific level with the cathode off-gas of low oxygen content at a high combustion temperature.
Thus, in the conventional art the fuel cell takes a large part in the power ratio of power plant, and when the fuel cell based on the power recovery by a turbo-charger is limited to a small power generation, the power generation is carried out only by the fuel cell. That is, the power plant output and the efficiency are entirely dependent on the fuel cell. In other words, no consideration is paid to reduction in the power ratio of the fuel cell in an integrated power plant and to the consequent reduction in the risk of the fuel cell as a product still under development, and also to needs for increasing the overall thermal efficiency of the plant over that of the conventional power plant. The risk due to the reliability of the fuel cell proper has been a large problem that will become a risk on the total plant investment.